This invention relates to removable control units of motor control centers and more particularly relates to a latch mechanism for retaining the control unit in position within the motor control center cabinet.
Motor control centers comprise a cabinet having a plurality of vertically arranged compartments open to a forward face of the cabinet. The cabinet is provided with a plurality of doors individual to each compartment for closing off the same. A plurality of electrical bus bars are mounted within the cabinet at the rear, the bus bars being oriented vertically to traverse the plurality of compartments. The cabinets are adapted to be mounted side by side with provisions for connecting the vertical bus bars of each cabinet to a common source of supply through horizontal busing extending between the cabinets. In some instances the motor control cabinets are made to be double sided or back-to-back wherein the cabinet is extended to the rear to accommodate a second set of compartments open to the opposite side of the cabinet as the first set of compartments, each set of compartments commonly sharing the vertically oriented bus bars which in this instance are centrally located within the cabinet.
The compartments each contain a removable control unit which may be slidably inserted or removed from the compartment when the respective door is open. The unit comprises a sheet metal structure which serves as a mounting panel for various motor control components as required. The cabinets are structured to enable the compartment sizes to be varied as required, customarily in six inch vertical increments, to provide the motor control center with the flexibility of performing a wide range of control applications. Each control unit is provided with a plurality of stab-on electrical connectors at the rear of the unit for making electrical connection between the bus bars and the unit by insertion of the unit within the compartment. In view of the large current carrying capacity of the bus bars, the spring pressure of the stab-on contacts creates a high insertion force for such plug-in electrical connection. Moreover, the weight and bulk of the larger control units increases the difficulty of inserting the control unit to make electrical connections to the bus bars.
The control units are also provided with latching means which engage with fixed members of the cabinet to prevent the unit from being unintentionally uncoupled from the bus bars. In some cases the latch means are spring loaded plungers or levers carried by the unit and which are biased laterally outwardly of the unit to engage the cabinet along the respective sides of the control unit. The plungers or levers may require the installer to physically retract them while inserting the control unit into the compartment, or may be provided with cam surfaces which enable the latch members to be cammed aside during insertion of the unit. Upon removal of the control unit from the cabinet, the operator is required to physically manipulate the latch means to release the same while pulling the unit outwardly to uncouple the stab-on connectors from the bus bars. Still another form of latch has quarter turn fasteners provided at two or more corners of the control unit or at the compartment opening which require the installer to rotate the latches with a tool to either latch or unlatch the devices. Latches of this type do not operate automatically upon insertion, and are preferably not utilized as the sole means of latching the unit within the compartment.
As mentioned above, the insertion force for the electrical connectors to the bus bars can be very high and to this end mechanisms have been provided in motor control center removable units or in the cabinets thereof to assist in the stab-on or withdrawal movements between the electrical connector and the bus bar. For example, it is known to pivotally mount a lever on a control unit such that one end of the lever engages with a portion of the cabinet upon partial insertion of the control unit into the compartment. Final movement of the control unit into the compartment or initial movement of the control unit out of the compartment, during which movement the electrical connections are either made or broken, is effected by pivoting the lever to a position flush with the front face of the control unit.